Where would I connect the Midi to CV gate (or other trigger signal) so that it triggers the whole envelope?
The Gate In of the 1st DH_EnvSeg module in a chain is the gate for the entire multi-stage envelope. It would typically be connected to a triggering signal such as the gate of a Midi to CV module. The Start Level defaults to 0, so the Start Level plug of the 1st module can be left unconnected unless you want the whole envelope to start at a level other than 0.